1. Field of the Invention
The invention relates to a sync separating circuit for extracting a sync signal of a video signal and, more particularly, to a sync separating circuit to extract a sync signal from a digital video signal.
2. Description of the Related Art
As shown in FIG. 1, according to a conventional sync separating circuit, a video signal is clamped, the clamped video signal is sliced by a predetermined level, and a sync signal is separated by the amplitude separation (for example, JP-A-6-253170).
That is, in FIG. 1, a video signal is supplied to an input terminal 51. The video signal from the input terminal 51 is supplied to a clamping circuit 52. In the clamping circuit 52, the video signal is clamped so that, for example, a sync chip level of the video signal is made constant. An output of the clamping circuit 52 is supplied to a slice circuit 53. A level that is slightly higher than the sync chip level of the video signal is set as a slice level th into the slice circuit 53. In the slice circuit 53, the level of the video signal is compared with the slice level th.
When the inputted video signal lies within a video period of time, the output level of the clamping circuit 52 is higher than the threshold level th. In a period of time of the sync signal, the output level of the clamping circuit 52 is lower than the threshold level th. Thus, the sync signal can be extracted from the inputted video signal. The sync signal which is outputted from the slice circuit 53 is taken out from an output terminal 54.
In recent years, to realize a small size, a light weight, and low costs of a set, a video signal processing circuit is digitized and arranged on an integrated circuit. A circuit for directly digitizing the video signal and separating a sync signal from the digital video signal is considered.
In case of extracting the sync signal from the analog video signal as mentioned above, the sync chip level of the video signal is clamped and sliced by a slice level that is slightly higher than the sync chip level, so that the sync signal can be separated. However, in the case where the video signal which is not clamped is directly digitized and processed, since the sync chip level fluctuates, it is difficult to accurately extract the sync signal.
Therefore, as shown in FIG. 2A, for example, there is considered a method whereby a minimum value S1 of the video signal is detected and a slice level formed on the basis of the minimum value S1 of the video signal is compared with the video signal, thereby extracting the sync signal. Since the sync chip level of the sync signal is equal to the minimum value of the video signal, if the minimum value of the video signal is detected and the slice level is formed as mentioned above, the slice level can be set to a level that is slightly higher than the sync chip level of the sync signal. Thus, the sync signal can be extracted.
However, as shown in FIG. 2B, particularly, in a reception signal at a place of a weak electric field or a reproduction signal from a tape in which the dubbing was repeated, there is a case where a noise N which drops to a low level is generated in the video signal. If the slice level is formed on the basis of a minimum value S2 of the video signal, the slice level is equal to or less than the sync chip level due to an influence by the noise N which drops lower as mentioned above, so that a problem occurs such that the sync signal cannot be extracted from the video signal.